French Patent No. 2 667 979 describes a method for adjusting a thermal overload cutout having bimetallic strips, a pickoff system, and a cutout mechanism which has an adjustment device. The position of the bimetallic strips is first matched to the pickoff system, and a preadjustment of the cutout mechanism is performed by setting a predefined cutout distance, the bimetallic strips are then acted upon by a specific current.
Adjustment methods and thermal overload cutouts of this kind are already known. Protective devices having delayed-action overload cutouts of this kind must, according to specifications, cut out within specific response limits (e.g. International Electratechnical Commission IEC 947-2, IEC 947-4-1). According to these specifications, a cutout must occur within 2 hours under a load equal to 1.2 times the preset current, and no cutout must occur within 2 hours of current application with 1.05 times the preset current. In devices having thermal overload cutouts, this is usually achieved by converting the current, via the electric heat, into a deflection of bimetallic strips, and adjusting to these bimetallic strips a mechanical system which contains a kickover function.
The effective deflection of the bimetallic strips depends on a variety of influencing factors, for example the current path resistance of the thermal overload cutout, the specific deflection of the bimetallic strips, the cutout force and resilience of the mechanical system, etc. Individual deflections thus exist for each protective device.
FIG. 2 shows a schematic sketch of a conventional thermal overload cutout. It contains three bimetallic strips 1, a mechanical pickoff system 2, and a cutout mechanism 3 having a compensation strip and an adjusting device 5. In the conventional adjustment method, it is assumed that protective devices of uniform design have an average effective deflection. This deflection travel is set as the distance y between the bimetallic strips and the mechanical shutoff point of the cutout mechanism (preadjustment), and the position of the bimetallic strips is then matched to the pickoff system (synchronization). The cutout time of the thermal overload cutout at a specific current is then measured. If the cutout time falls outside a predefined time window determined by the current response limits, the distance must be corrected based on the time deviation. The cutout time measurement is repeated after the protective device has cooled to ambient temperature.
In order to ensure that response limits are within specifications, randomly sampled measurements of the response limits are made. If the results show a trend toward a change in the response limits, the latter are corrected toward the middle of the specified response limits by shifting the time window when the cutout time is checked.